Polyethylenes of various densities have been prepared and converted into film characterized by excellent tensile strength, high ultimate elongation, good impact strength, and excellent puncture resistance. These properties together with toughness are enhanced when the polyethylene is of high molecular weight. However, as the molecular weight of the polyethylene increases, the processability of the resin usually decreases. By providing a blend of polymers of high molecular weight and low molecular weight, the properties characteristic of high molecular weight resins can be retained and processability, particularly extrudability (a characteristic of the lower molecular weight component) can be improved.
The blending of these polymers is successfully achieved in a staged reactor process similar to those described in U.S. Pat. Nos. 5,047,468 and 5,149,738. Briefly, the process is one for the in situ blending of polymers wherein a high molecular weight ethylene copolymer is prepared in one reactor and a low molecular weight ethylene copolymer is prepared in another reactor. The process typically comprises continuously contacting, under polymerization conditions, a mixture of ethylene and one or more alpha-olefins with a catalyst system in two gas phase, fluidized bed reactors connected in series, said catalyst system comprising: (i) a supported magnesium/titanium based catalyst precursor; (ii) one or more aluminum containing activator compounds; and (iii) a hydrocarbyl aluminum cocatalyst, the polymerization conditions being such that an ethylene copolymer having a melt index in the range of about 0.1 to about 1000 grams per 10 minutes is formed in the high melt index (low molecular weight) reactor and an ethylene copolymer having a melt index in the range of about 0.001 to about 1 gram per 10 minutes is formed in the low melt index (high molecular weight) reactor, each copolymer having a density of about 0.860 to about 0.965 gram per cubic centimeter and a melt flow ratio in the range of about 22 to about 70, with the provisos that:
(a) the mixture of ethylene copolymer matrix and active catalyst precursor formed in the first reactor in the series is transferred to the second reactor in the series; PA1 (b) other than the active catalyst precursor referred to in proviso (a), no additional catalyst is introduced into the second reactor. PA1 (a) the particulate precursor has a particle size distribution span of no greater than about 1.5 as introduced into the first reactor in the series; PA1 (b ) ethylene is introduced into each reactor; PA1 (c) optionally, an alpha-olefin having at least 3 carbon atoms is introduced into at least one reactor; PA1 (d) the mixture of ethylene polymer matrix and active catalyst formed in the first reactor in the series is transferred to the subsequent reactors in the series; and PA1 (e) the polymerization conditions in each reactor are such that a high molecular weight polymer is formed in at least one reactor and a low molecular weight polymer is formed in at least one other reactor wherein the ratio of molecular weights of high molecular weight polymer to low molecular weight polymer in the final blend is at least about 8:1. PA1 A preferred embodiment of the foregoing process comprises contacting a magnesium/titanium based catalyst system including a supported, spray dried, or precipitated particulate precursor with ethylene or a mixture of ethylene and one or more alpha-olefin comonomers having 3 to 12 carbon atoms in each of two reactors connected in series, in the gas phase, under polymerization conditions, with the provisos that: PA1 (a) the particulate precursor has a particle size distribution span of no greater than about 1.2 as introduced into the first reactor in the series; PA1 (b) the mixture of ethylene polymer matrix and active catalyst formed in the first reactor in the series is transferred to the second reactor in the series; PA1 (c) no additional catalyst is introduced into the second reactor; PA1 (d) in the reactor in which a high molecular weight polymer is made: PA1 (e) in the reactor in which a low molecular weight polymer is made: PA1 (f) the polymerization conditions in each reactor are such that a high molecular weight polymer is formed in at least one reactor and a low molecular weight polymer is formed in at least one other reactor wherein the ratio of molecular weights of high molecular weight polymer to low molecular weight polymer in the final blend is at least about 20:1. PA1 Span=(D.sub.90 -D.sub.10).div.D.sub.50 wherein D is the median particle size as measured by diameter at the 90th, 10th, or 50th percentile of the distribution. PA1 (a) a solid particulate catalyst precursor having the formula Mg.sub.d Ti(OR).sub.e X.sub.f (ED).sub.g wherein R is an aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms or COR' wherein R' is a aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms; each OR group is the same or different; X is independently chlorine, bromine or iodine; ED is an electron donor; d is 0.5 to 56; e is 0, 1, or 2; f is 2 to 116; and g is 1.5d+2; PA1 (b) at least one modifier having the formula BX.sub.3 or AlR.sub.(3-e) X.sub.e wherein each R is alkyl or aryl and is the same or different, and X and e are as defined above for component (a) PA1 (c) a hydrocarbyl aluminum cocatalyst.
While the in situ blends prepared as above and the films produced therefrom are found to have the advantageous characteristics heretofore mentioned, the commercial application of these granular bimodal polymers for high clarity film applications is frequently limited by the level of gels obtained. Particle size distribution and flow characteristics studies indicate that the gas phase resins having an average particle size (APS) of about 400 to about 600 microns exhibit significant compositional, molecular, and theological heterogeneities. When such a granular resin is compounded, for example, with a conventional twin screw mixer in a single pass, and the resulting pellets are fabricated into film, the film exhibits a high level of gels ranging in size from less than about 100 microns to greater than about 500 microns. These gels adversely effect the aesthetic appearance of the product. The gel characteristics of a film product are usually designated by a subjective scale of Film Appearance Rating (FAR) varying from minus 50 (very poor; these films have a large number of large gels) to plus 50/plus 60 (very good; these films have a small amount of, or essentially no, gels). The FAR of the single pass film product mentioned above is generally in the range of about minus 50 to about minus 10/0. For commercial acceptability, the FAR should be plus 20 or better.
Three suggestions have been made for improvement of the FAR, i.e., removal of the fraction containing the larger resin particles so as to remove the suspected source of the large gels; making the components of the resin particle more similar to facilitate their mixing within the resin particle; and the use of longer residence times in the extruder to achieve more efficient mixing of the resin particles. Unfortunately, removal of the larger resin particles was found to increase the size and number of gels in the film; the use of similar components improved the FAR, but did not provide the desired increase in the end-use properties of the resin; and the longer residence time in the extruder proved to be logistically unacceptable and prohibitively expensive.